A capacitor microphone incorporates an impedance converter such as an FET (Field Effect Transistor) because of the very high impedance of a microphone unit. The capacitor microphone normally uses a phantom power source. A microphone sound signal is output via a balanced shield cable for the phantom power source.
A microphone case (in a handheld microphone, a microphone grip) is provided with a 3-pin type output connector for a connection with the balanced shield cable (see, for example, Japanese Patent Application Publication No. H11-341583). The output connector is defined in EIAJ RC-5236 “Latch Lock Type Round Connector for Acoustic Equipment”. The configuration of the output connector will be described with reference to FIGS. 5 to 7.
FIG. 5 is a sectional view showing an output connector installed in a microphone case. FIG. 6 is a front view showing the connector extracted from the microphone case. FIG. 5 is a sectional view of the output connector taken along line V—V in FIG. 6. FIG. 7 is a plan view of the output connector.
The output connector 10 comprises a disk-shaped connector base 11 consisting of an electric insulator such as a PBT (polybutadiene terephthalate) resin. Three pins, that is, a first pin E for earthing, a second pin SH on a signal hot side, and a third pin SC for a signal cold side, are for example, pressed in the connector base 11 so as to penetrate it.
In connection with a handheld microphone, as shown in FIG. 5, the output connector 10 is installed in a connector housing cylinder 20 screwed at an end of the microphone grip. The microphone grip, including the connector housing cylinder 20, consists of metal such as brass. The microphone grip acts as a shield case for incorporated electric parts.
A male thread 12 is formed in the connector base 11 so as to electrically connect the first pin E for earthing to the connector housing cylinder 20 together. The male thread 12 is housed in a thread housing hole 13 drilled in the connector base 11 in a radial direction. The connector base 11 is provided with an earth terminal plate 14 having a female thread 14a in which the male thread 12 is screwed in the thread housing hole 13.
As shown in the plan view in FIG. 7, the earth terminal plate 14 and the first pin E for earthing are electrically connected together via a connection fixture 15. As shown in FIG. 5, a driver (not shown) is used to rotate the female thread 14a through a hole 21 drilled in the connector housing cylinder 20. The male thread 12 is thus abutted against the periphery of the hole 21.
Thus, the first pin E for earthing and the connector housing cylinder 20 are electrically connected together via the earth terminal plate 14 and the connection fixture 15. Further, as shown in FIGS. 6 and 7, a plate spring 16 is connected to the first pin E for earthing; the plate spring 16 contacts an inner surface of the connector housing cylinder 20. The plate spring 16 may electrically connect the first pin E for earthing and the connector housing cylinder 20 together.
While a microphone cable drawn from a phantom power source (not shown) is connected to the output connector 10, when an intense electromagnetic wave is applied to the microphone or the microphone cable, the electromagnetic wave may enter the microphone through the output connector 10. In this case, the electromagnetic wave is demodulated by an impedance converter and output by the microphone as noise of an audible frequency.
To prevent an electromagnetic wave from entering the microphone through the output connector 10, a conventional technique for connecting capacitors between the first pin E for earthing and the second pin SH for the hot side and between the first pin E for earthing and the third pin SC for the cold side; the capacitors operate so as to short-circuit high frequencies. The conventional technique further connects the second pin SH for the hot side and the third pin SC for the cold side to a microphone case such as the connector housing cylinder 20 via an inductor used to inhibit the entry of high frequencies.
This conventional technique can appropriately inhibit the entry of normal broadcasting waves, for example, electromagnetic waves of HF, VHF, UHF, or the like. However, the recent prevalence of cellular phones or the like increases opportunities to use electromagnetic waves of higher frequencies near the microphone.
Three pins E, SH, and SC are penetratingly provided in the output connector 10. However, the area between the pins is partly unshielded, so that electromagnetic waves can enter the connector through this part. Further, the contacted part of the output connector 10 to the connector housing cylinder 20 is a part of the plate spring 16, attached to the male thread 12 for fixation and/or the first pin E for earthing as described above. Therefore, this contact part has a high impedance for high frequencies and is not sufficiently earthed for high frequencies. Consequently, even if the capacitors and inductor are connected as described above so as to inhibit high frequencies, this is not sufficiently effective on high frequencies used in cellular phones or the like.
Further, the capacitors have a maximum rated voltage above which themselves may be destroyed. When totally destroyed, the capacitors lose their functions. However, if the level of the destruction is low, the functions of the capacitors are maintained to some degree. This may in turn cause noise.